1. Field of the Invention
In optical wavelength-division multiplexing communication it is necessary to control both wavelength and level of each transmitted signal light in order to receive a transmitted optical signal in an optimal condition on the receiving side.
Furthermore, recently relay distances have been increased by installing repeaters with optical amplifiers in a transmission path between terminal stations.
The present invention relates to both wavelength control and level control of terminal equipment in a communication system configuration having a repeater with such an optical amplifier.
2. Description of the Related Art
FIGS. 1A and 1B show the pre-emphasis (explained below) in the case where wavelength-division multiplexing (WDM) transmission is made by connecting a plurality of optical amplifiers in series, and the optical signal-to-noise ratio (OSNR) after transmission.
In FIGS. 1A and 1B, .lambda.1 to .lambda.8 represent the wavelengths of the signal lights of an optical signal, and the heights of the wavelengths of the signal lights .lambda.1 to .lambda.8 represent the strength of each signal light.
In FIGS. 1A and 1B, an optical signal comprises a plurality of signal lights of different wavelengths, represented by .lambda.1 to .lambda.8.
In a transmission path 1 optical amplifiers 2-1 to 2-n used as repeaters are shown.
FIG. 1A shows the gain characteristic of the transmission path 1, in which the losses of optical amplifiers 2-1 to 2-n in the transmission path 1 and the transmission path itself are totaled, and the optical signal-to-noise ratio (OSNR) for each signal light, being a ratio in terms of the electric power of the signal light to that of the noise light.
When signal lights .lambda.1 to .lambda.8 with the same level are transmitted from the transmitting side, differences in levels occur between the signal lights due to the gain characteristic of the transmission path 1.
For this reason, as shown in FIG. 1B, to compensate for the gain characteristic of an optical repeater in the transmission path, the level of each signal light is changed on the transmitting side, by which the fluctuation in OSNR is suppressed when the signal lights are received.
This technology is called pre-emphasis, and is widely used.
The pre-emphasis shown in FIG. 1B is adjusted and fixed in the initial stages of the operation of a system.
However, the level of light input to the optical amplifiers 2-1 to 2-n fluctuates, and the gain of the entire transmission path 1 changes due to a fluctuation of the losses in the transmission path 1 during the operation of a system, caused by such as the insertion in the transmission path 1 of a new cable for the repair of a failed section, the insertion in the transmission path 1 of a repeater, the deterioration with the lapse of time of an optical fiber in the transmission path 1, etc., and the gain also changes due to the deterioration with the lapse of time of the optical amplifiers 2-1 to 2-n.
As a result, the gain characteristic of the entire transmission path changes, and the fluctuation of the OSNR cannot be compensated for by a fixed pre-emphasis method.
FIGS. 2A to 2C show the concept of a spectrum change of the signal lights with the lapse of time, and FIGS. 3A to 3C show the experiment results of the spectrum change of the signal lights with the lapse of time.
FIGS. 2A and 3A, FIGS. 2B and 3B, and FIGS. 2C and 3C show the original condition, after the lapse of a certain time, and after the lapse of a further certain time, respectively.
As shown in FIGS. 2A to 2C and FIGS. 3A to 3C, the fluctuation of the OSNR of each signal light increases as time elapses.
Particularly, this becomes a serious problem in the case of a submarine cable system used for long distance transmission.
As means to solve a problem like this, a technology with which the pre-emphasis can be changed on the transmitting side based on the spectrum of the receiving side, is shown in Laid-open Patent Publication No.8-321824.
More specifically, the spectrum of the wavelengths of each of the signal lights is measured in a receiving terminal station, the result of the measurement is overlaid on an optical signal or transmitted by being put in the vacant area of a header of the synchronous digital hierarchy (SDH), etc. to the transmitting terminal station using a downward line, and the amount of pre-emphasis is measured in the transmitting terminal station.
Although it is necessary to control the pre-emphasis in order to improve the OSNR on the receiving side, the gain fluctuation of the transmission path caused by the fluctuation of the losses due to the insertion in the transmission path of a new cable or the deterioration with the lapse of time, cannot be coped with by a fixed pre-emphasis method.
Although there is a technology to automatically control the pre-emphasis in the transmitting terminal station based on spectrum information sent from the receiving terminal, in order to cope with this situation, in this technology attention is paid only to the pre-emphasis control.
Although it is necessary to adjust the pre-emphasis in order to improve the OSNR of the wavelength-division multiplexed signal lights sent from the transmitting terminal station, both the wavelengths and output levels of the signal lights of the optical signal vary depending on both temperature and driving current value, since a signal light for each wavelength of the optical signal from the transmitting terminal station is output by a laser.
Particularly, when the high density of wavelength-division multiplexing is increased (the frequency of multiplexing), and it becomes necessary to strictly control the wavelength of each signal light, both wavelength fluctuation and level fluctuation occur if the pre-emphasis is controlled only based on information sent from the receiving terminal station.